SPF is an easy and lightway way to filter out large amount of spam messages. The exim package, that comes bundled with cPanel is compiled with SPF and SRS support. Although tagged experimental, both are – in my experience – stable and production ready. One problem remains – though SPF support can be enabled from cPanel’s interface – it is currently not possible to turn SRS on, which can screw up forwarding pretty bad. Fortunately, enabling it manually is quite easy and only requires modification of two router (virtual_aliases_nostar and virtual_aliases) and adding third one in exim’s configuration file.